a) Field of the Invention
The present invention relates to a drive assembly of an electric bicycle, and more particularly to a drive assembly of an electric bicycle which is able to assist pedaling the bicycle with feet, wherein a displacement of a crank shaft pulled by a bicycle chain is used to acquire a torque, such that an output power of a tubular motor set can be controlled, thereby achieving a purpose of assisting pedaling the bicycle with feet.                b) Description of the Prior Art        
A bicycle is not only a vehicle of transportation, but also sport equipment for spending a leisure time. The bicycle is primarily operated by pedaling with a rider's feet to drive a chain wheel set that links to a bicycle chain, thereby driving a rear wheel to rotate, as a power for moving the bicycle forward. However, the road condition that an ordinary bicycle travels is not always the same; sometimes the road is smooth, and sometimes the road is a ramp. When riding the bicycle for a long distance, some people with an inferior physical strength, such as elders, women or children, may be incapable of completing the whole journey. Therefore, there are electric bicycles to solve this kind of problem.
The existing electric bicycle is operated primarily by an electric motor which generates a power to drive the chain wheel set to move a bicycle chain that links to and rotates a rear wheel to move the bicycle forward. The related technical means can be referred to in prior arts of U.S. Pat. No. 6,230,586, U.S. Pat. No. 8,418,584, EP 11164464.7, and TW 201238831A1.
As mentioned above, in addition to providing for manually pedaling a bicycle, the electric bicycle can even allow the rider to choose using electricity as a power source for moving the bicycle forward. Nevertheless, the problem lies in that the design concept of the ordinary electric bicycle does not take the ability of pedaling the bicycle with feet into consideration, but only allows the rider to choose one operation mode for driving the bicycle forward between the manual pedaling and the electric power. The prior arts U.S. Pat. No. 6,230,586, U.S. Pat. No. 8,418,584, EP 11164464.7, and TW 201238831A1 also belong to this design concept. Yet, for a true lover of riding a bicycle, it will basically make no difference from riding a motorcycle if the bicycle can be only ridden by the electric power, and the rider will not enjoy the pleasure of pedaling a bicycle and the sense of accomplishment in conquering a physical limit at all.
Accordingly, there are vendors who utilize a detected rotational torque of the crank shaft of the bicycle to control the output power of the electric motor, serving as a power to assist pedaling the bicycle with feet. The related technical applications can be referred to in prior arts TW 451316 and TW 453843. However, the common idea of these prior arts is the detection of the rotational torque of the crank shaft, which is not able to accurately detect the ability of pedaling a bicycle of a rider. To put it simply, although the rotation of the crank shaft is indeed related to the ability of pedaling a bicycle, the conventional method for detecting the torque cannot detect whether the rider is difficult in pedaling the bicycle, and even the rotational torque and the rotational speed of the crank shaft are under a same condition, the physical burden of pedaling the bicycle is not the same for all the riders. This is the blind spot of the existing method that uses the torque detection to determine the ability of pedaling the bicycle.
Furthermore, most of the existing torque detection devices and speed detection devices are hung externally and therefore can be damaged easily by collision or an external force. In addition, there are some shortcomings when hanging these devices externally; for example, the accuracy of detection can be easily affected by other external factors mixing with other parameters, during the detection process.